In multi-compressor air conditioning systems, such as may be installed on a roof-top (identified as roof-top units even though frequently positioned in some other convenient location but still referred to as roof-top units regardless of location), an active cooling mode is entered into by the system when a cooling demand is called for by a controller in response to a sensed temperature. Such a multi-compressor air conditioning system may or may not utilize an economizer. Briefly, economizers are used to provide energy savings by reducing the need for mechanical cooling by the compressors in the multi-compressor system by taking advantage of ambient temperature and humidity when it is lower than that of interior air. Economizers may also provide a reduction of indoor CO2 levels. Economizers also are controlled by the controller, which monitors installed sensors positioned at various locations inside the rooftop unit, inside the interior region and outside the building. The installed sensors monitor temperature and humidity levels inside and outside the building, as well as CO2 sensors to monitor conditions inside the building. This information is sent to the controller which then determines system operation, adjusting economizer operation that the proper configuration to control outside air input into the building, based on sensed conditions.
The controller may be provided with a setting that disables economizer logic and operation, which is useful when no economizer is installed. However, the controller may be programmed without such a setting, so that the programmed controller may be used universally, i.e. shipped from the factory, without regard as to whether the multi-compressor system will be used with an economizer. When programmed in this manner, the controller first checks to determine whether an economizer is installed, before entering into the active cooling mode.
Based on sensed conditions in both the interior region and outside, when the controller determines cooling is needed and either determines that conditions are not suitable for activation of the economizer, or determines that an economizer is not installed, the controller is programmed to stage compressor operation individually and in seriatim to control the indoor supply air temperature so that the actual sensed interior region air temperature at a preselected location reaches the interior region temperature set point. While this approach works well, it does have a few disadvantages. For example, once the controller activates a compressor, it is not known immediately how much the sensed interior region temperature will drop upon compressor activation.
The controller is programmed to initialize the operation of the first compressor and operate it for a preselected, preprogrammed period of time, usually 3½ minutes, while monitoring the sensed interior region temperature and comparing it to the interior region temperature set point before implementing further action. At the end of this preprogrammed period of time, the controller, using a Cooling Control Offset calculation procedure, an algorithm programmed into the controller, may initiate start-up of the next compressor in a multi-compressor system. This algorithm may vary from system to system, but evaluates the need for an additional compressor to further reduce the interior region temperature. This process is repeated after each additional compressor is brought online, until the actual sensed interior region temperature matches, or is within a predetermined tolerance range, of the interior region temperature set point. Of course, if the load is high, it could take a substantial amount of time to initiate operation of all compressors in the multi-compressor system. For example, in a system having six compressors, that is, a six stage unit, it may take in excess of twenty minutes to initiate operation of all six compressors in high load conditions. Such a time delay also postpones the time for the actual sensed interior region temperature to match the interior region temperature set point. What is needed is a system that maximizes compressor usage in a multi-compressor system to achieve matching of the actual sensed interior region temperature to the interior region temperature set point as quickly as possible.